Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process which includes liquid isomerization in an external downcomer reaction loop. More particularly the invention relates to a slurry Fischer-Tropsch type of hydrocarbon synthesis process, in which the synthesized hydrocarbon slurry liquid in the synthesis reactor is circulated through at least one external downcomer reactor, in which it reacts with hydrogen in the presence of a hydroisomerization catalyst, and preferably a monolithic catalyst, to hydroisomerize the liquid and reduce its pour point. The liquid then passes back into the synthesis reactor.
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas, which comprises a mixture of H2 and CO, up into a hot reactive slurry in a hydrocarbon synthesis reactor. The slurry comprises synthesized hydrocarbons which are liquid at the synthesis reaction conditions and in which is dispersed a particulate Fischer-Tropsch type of catalyst. The H2 and CO react in the presence of the catalyst and form hydrocarbons. The hydrocarbon liquid is continuously or intermittently withdrawn from the reactor and pipelined to one or more downstream upgrading operations. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. The downstream upgrading includes fractionation and conversion operations, typically comprising hydroisomerization, in which a portion of the molecular structure of at least some the hydrocarbon molecules is changed. It would be an improvement if the synthesized hydrocarbon slurry liquid could be hydroisomerized to reduce its pour and melt points, which make it more transportable by pipeline, before it is transferred to downstream operations.
The invention relates to a slurry Fischer-Tropsch type of hydrocarbon synthesis process, in which a portion of the synthesized hydrocarbon slurry liquid is passed out of the synthesis reactor and into at least one external downcomer reactor, in which it reacts with hydrogen in the presence of a hydroisomerization catalyst, and preferably a monolithic hydroisomerization catalyst, to hydroisomerize the liquid, which is then passed back into the three-phase slurry (main slurry body) in the synthesis reactor. The slurry liquid, which comprises synthesized hydrocarbons that are liquid at the synthesis reaction conditions, comprises mostly normal paraffins and the hydroisomerization reduces its pour and melt points, thereby making it more pumpable and pipelineable. By downcomer reactor is meant that all or most of the slurry circulation between it and the synthesis reactor is achieved by density-driven hydraulics, in which the density of the downflowing slurry is greater than in the synthesis reactor. Slurry densification is achieved by removing at least a portion of the gas bubbles from the slurry, thereby densifying the slurry, before it is passed into the downcomer reactor. The one or more downflow reactors may each be a simple, substantially vertical, hollow fluid conduit or pipe. The process comprises contacting hot slurry from the main slurry body, with means for removing gas bubbles, and preferably both gas bubbles and at least a portion of the particulate solids from the slurry liquid which, along with a hydrogen treat gas, is then passed out of the synthesis reactor and down into the one or more external downcomer reactors. The hydroisomerization catalyst is located in the interior of the downcomer reactor and comprises the hydroisomerization reaction zone. This hydroisomerized hydrocarbon liquid of reduced pour point is then passed back into the main slurry body in the synthesis reactor. Thus, the synthesized hydrocarbon liquid is passed out of the synthesis reactor, down into and through the interior of the one or more external downcomer reactors and back into the synthesis reactor. The downcomer reactor is in fluid communication with the main slurry body inside the synthesis reactor, via upper and lower conduit portions opening into respective upper and lower portions of the synthesis reactor. This enables hydroisomerization of the slurry liquid (i) in an external reaction loop which depends from, and is therefore part of, the synthesis reactor and (ii) while the synthesis reactor is producing hydrocarbons, but without interfering with the hydrocarbon synthesis reaction. The concentration of hydroisomerized hydrocarbon liquid in the synthesis reactor continues to increase until equilibrium conditions are reached. When the reactor reaches equilibrium, it is possible for the slurry liquid being removed from it to comprise mostly hydroisomerized hydrocarbons of reduced pour point. In some cases, no further hydroisomerization of the liquid hydrocarbon product withdrawn from the hydrocarbon synthesis reactor is necessary. Thus, the process of the invention will reduce and in some cases even eliminate the need for a separate, stand-alone hydroisomerization reactor and associated equipment, downstream of the synthesis reactor. If a downstream hydroisomerization reactor is needed, it will be smaller than it would be if the synthesized hydrocarbon liquid passed into it was not at least partially hydroisomerized. While all of the hydroisomerized hydrocarbon liquid is typically returned back into the main slurry body with which it mixes, in some embodiments a portion of the hydroisomerized liquid will be passed from the downcomer reactor directly to downstream operations.
Hydroisomerizing the slurry liquid in one or more external loops permits the use of heat exchange means associated therewith to adjust the hydroisomerization temperature to be different (e.g., higher) from that in the synthesis reactor. A higher hydroisomerization temperature enables the use of a less expensive, non-noble metal hydroisomerization catalyst. The gas bubble and preferably the slurry gas bubble and particulate solids removal means is preferably located in the main slurry body and may comprise the same or separate means. While various filtration means may be used to separate the slurry liquid from at least a portion of the catalyst and any other particles, before the slurry is passed down into the hydroisomerization zone, in the practice of the invention the use of filtration means may be avoided by using known slurry solids reducing means that do not employ filtration. Gas bubble and solids removal means suitable for use with the present invention are known and disclosed in, for example, U.S. Pat. Nos. 5,866,621 and 5,962,537, the disclosures of which are incorporated herein by reference. Simple gas bubble removing means are disclosed in U.S. Pat. Nos. 5,382,748; 5,811,468 and 5,817,702, the disclosures of which are also incorporated herein by reference. Removing gas bubbles from the slurry densities it and, if properly employed in connection with feeding the densified slurry down into and through the downcomer reactor (e.g., the slurry is densified sufficiently vertically above the external hydroisomerization zone), provides a density-difference driven hydraulic head to circulate the slurry from inside the synthesis reactor, down into and through the external downcomer reactor and back into the synthesis reactor. Removing gas bubbles from the slurry prior to hydroisomerization also reduces the CO and water vapor content of the flowing fluid, which could otherwise react with the hydroisomerization hydrogen and also adversely effect the hydroisomerization catalyst. A monolithic hydroisomerization catalyst having a minimal solid cross-sectional area perpendicular to the flow direction of the fluid, minimizes the pressure drop of the fluid flowing down and across the catalyst surface. Removing catalyst and other solid particles, such as inert heat transfer particles, from the slurry upstream of the hydroisomerization zone, reduces scouring of the monolithic catalyst and plugging of the hydroisomerization reaction zone.
In a broad sense, the process of the invention comprises a slurry Fischer-Tropsch hydrocarbon synthesis process, in which a portion of the hydrocarbon slurry liquid is removed from the main slurry body in the hydrocarbon synthesis reactor, reduced in gas bubble content and passed down into and through a hydroisomerization zone in a downcomer reactor external of, and in fluid communication with, the synthesis reactor, in which it reacts with hydrogen in the presence of a hydroisomerization catalyst, at reaction conditions effective to hydroisomerize at least a portion of the hydrocarbon liquid and produce a hydroisomerized hydrocarbon liquid of reduced pour point, with at least a portion of the hydroisomerized passed back into the synthesis reactor. Preferably at least a portion of both gas bubbles and particulate solids are removed from the slurry before it contacts the hydroisomerization catalyst. In a still further embodiment, the invention comprises a hydrocarbon synthesis process which includes hydroisomerizing hydrocarbon liquid produced by the synthesis reaction while the hydrocarbon liquid is being produced from a synthesis gas, the process comprising the steps of:
(a) passing a synthesis gas comprising a mixture of H2 and CO into a slurry body comprising a three-phase slurry in a slurry Fischer-Tropsch hydrocarbon synthesis reactor, in which the slurry comprises gas bubbles and a particulate hydrocarbon synthesis catalyst in a slurry hydrocarbon liquid;
(b) reacting the H2 and CO in the presence of the catalyst at reaction conditions effective to form hydrocarbons, a portion of which are liquid at the reaction conditions and comprise the slurry hydrocarbon liquid;
(c) contacting a portion of the slurry from the slurry body with means for removing gas bubbles, to form a densified slurry hydrocarbon liquid reduced in gas bubbles whose density is greater than that of the slurry comprising the slurry body in the synthesis reactor;
(d) passing a hydrogen treat gas and the densified hydrocarbon liquid formed in (iii) into a hydroisomerizing zone in one or more downcomer reactors external of, in fluid contact with and depending from the synthesis reactor, in which they react in the presence of a preferably monolithic hydroisomerization catalyst to form a hydrocarbon liquid of reduced pour point, and
(e) passing all or a portion of the pour point reduced liquid back into the synthesis reactor, wherein it mixes with the main slurry body therein.
While the liquid is being synthesized and hydroisomerized in the synthesis reactor, a portion is continuously or intermittently withdrawn and sent to downstream operations.